1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell and an energy storage which is charged by the power generation current of the fuel cell, and assists the output of the fuel cell. Further, the present invention relates to a method of adjusting the amount of electrical energy charged in the energy storage. In particular, the present invention relates to a fuel cell system and a method of adjusting the amount of electrical energy charged in an energy storage of the fuel cell system in which when power generation of the fuel cell system is stopped, a scavenging process is performed for an anode system in preparation for starting operation of the fuel cell system at low temperature such as the temperature below the freeing point.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces an electrochemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as the air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for inducing an electrochemical reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. The water is also retained at the anode, e.g., due to the back diffusion from the cathode or high humidification of the fuel gas.
If the water is excessively present at any of the electrodes, water clogging may occur. In view of the above, in the fuel cell system, a technique of the scavenging process as disclosed in Japanese Laid-Open Patent Publication No. 2003-331893 is proposed. In the scavenging technique, when operation of the fuel cell system is started or finished, an oxygen-containing gas is supplied to the anode in addition to the cathode for removing the water produced in the power generation from the membrane electrode assembly or the separators of the fuel cell.
In order to perform the scavenging process for the anode and/or the cathode, an energy source other than the fuel cell is required. Thus, in the fuel cell system, an energy storage such as a capacitor or a battery is mounted for assisting the output of the fuel cell.
When an ignition switch is turned off (power generation stop request), in the fuel cell system, the scavenging process is performed using the electrical energy stored in the energy storage in preparation for the next start-up operation of the fuel cell system, before stopping operation of the fuel cell system.
However, after the fuel cell system is stopped, if it becomes necessary to start operation of the fuel cell system at low temperature such as the temperature below the freezing point due to the decrease in the outside air temperature, it has been found that, in some cases, it becomes difficult to restart operation of the fuel cell system at low temperature such as the temperature below the freeing point. For example, this is caused by the decrease in the remaining electrical energy stored in the energy storage since the energy is consumed in the scavenging process.
Further, in the case where operation of the fuel cell system is started at low temperature such as the temperature below the freezing point, before the fuel cell is warmed up, the ignition switch may be turned off by a driver. Therefore, if operation of the fuel cell system is stopped after operation of the fuel cell system is started at the temperature below the freezing point, and power generation is performed for a short period of time, stated otherwise, if operation of the fuel cell system is stopped by the driver's operation in a short period of time after operation of the fuel cell is started at the temperature below the freezing point, it has been found that, in some cases, the fuel cell system becomes unstable unless a long scavenging process is performed, e.g., due to the insufficient activity of the electrolyte membrane. That is, it has been found that if operation of the fuel cell system is stopped after temporal start-up at low temperature, in some cases, it becomes very difficult to restart operation of the fuel cell system at low temperature such as the temperature below the freezing point, due to the further decrease of the remaining electrical energy in the energy storage.